1. Field of the Invention
The invention relates to compression ignition engines and, more particularly, relates to a method and apparatus for electronically controlling the injection of pilot fuel into such an engine.
2. Discussion of the Related Art
Recent years have seen an increased demand for the use of gaseous fuels as a primary fuel source in compression ignition engines. Gaseous fuels such as propane or natural gas are considered by many to be superior to diesel fuel and the like because gaseous fuels are generally less expensive, provide equal or greater power with equal or better mileage, and produce significantly lower emissions. This last benefit renders gaseous fuels particularly attractive because recently enacted and pending worldwide regulations may tend to prohibit the use of diesel fuel in many engines. The attractiveness of gaseous fuels is further enhanced by the fact that existing compression ignition engine designs can be readily adapted to burn gaseous fuels.
One drawback of gaseous fuels is that they exhibit significantly higher ignition temperatures than do diesel fuel, oil, and other liquid fuels traditionally used in compression ignition engines so that the temperature of gaseous fuels does not increase sufficiently during operation of standard compression ignition engines for self-ignition. This problem is overcome by injecting limited mounts of pilot fuel, typically diesel fuel, into the cylinders of the engine in the presence of a charge of a compressed gaseous fuel/air mixture in the combustion chamber. The pilot fuel ignites upon injection and bums at a high enough temperature to ignite the gaseous fuel charge.
The ignition characteristics of gaseous fuels vary markedly with engine operating conditions. For instance, changes in air/fuel ratio and density in the combustion chamber, occurring with changes in engine load, affect the combustion characteristics of the charge in the combustion chamber. Relatively lean charges (those exhibiting a high air/fuel ratio) are more difficult to ignite and take longer to burn than relatively rich charges (those exhibiting a relatively low air/fuel ratio). Relatively lean charges thus require that greater quantities of pilot fuel be injected at an advanced timing to obtain optimum primary fuel ignition, wherein "optimum" ignition is understood to those skilled in the art to achieve a desired balance of efficiency, power, and emissions. Injection characteristics are also affected by engine speed because ignition delays occurring during any combustion event have a greater impact at high engine speeds than at low engine speeds. These problems could be at least partially alleviated by always injecting sufficient pilot fuel to assure adequate primary fuel charge ignition, even under worst-case conditions. This solution is, however, undesirable because pilot fuel combustion produces undesired emissions the production of which should be minimized.
The above problems have not been sufficiently addressed by prior art pilot injection systems the injectors of which typically vary neither pilot fuel injection quantity nor timing. Those control schemes which have been proposed to alleviate these problems have met with only partial success. For instance, a method has been proposed in U.S. Pat. No. 4,831,993 to Kelgard (the Kelgard patent) to vary pilot injection timing. However, the method proposed in the Kelgard patent varies timing only with engine speed and does not vary injection quantity at all and thus has at best a limited impact on primary charge ignition and pilot charge optimization.